1. Field of the Invention
The present invention relates to a new fiber optic Sagnac interferometer for measuring rotation rates.
2. Description of the Prior Art
Fiber optic Sagnac interferometers, also known as fiber optic gyros (FOGs), are recognized devices for sensing rotational movements of a body, such as a projectile in inertial space. Such devices are found in various configurations and employ differing signal evaluation principles. A fiber optic gyro usually comprises an annular light path formed by means of optical wave guides, an optical phase modulator at one end of the path, optical couplers for coupling light into the light path from a light source and coupling out modulated light (such light carrying rate of rotation information) to a photodetector, and an electronic circuit arrangement for evaluating the detector signals and imposing a specified modulation upon the phase modulator.
Fiber optic gyro of principal interest to the present invention make use of a periodic phase shift at the modulator, such period corresponding to twice the light transit time in the annular light path, and an unmodulated light source of constant light power. When rotational movement of the annular light path (i.e., the ring) occurs, the photodetector is acted upon by a modulated light intensity that includes a component having the frequency of the modulation signal. The amplitude of this signal is a measure of the magnitude of the rate of rotation, while the phase relationship relative to the modulation phase provides information on the direction of rotation. By employing suitable carrier frequency modulation processes (e.g. synchronous demodulation with the modulation frequency as reference), it is possible to obtain a signal that is single-valued with respect to the direction of rotation and has a nonlinear amplitude-to-rotation rate relationship.
The so-called "readout" signal can either be directly evaluated as a signal carrying rotation rate information (open loop process) or may serve to supply the phase modulator with additional signals, via a control device, to compensate the effect of the rotation-generated phase shift (Sagnac phase) with an additional optical phase shift. This additional phase shift is likewise a direct measure of the rotation rate (closed loop process). The latter process requires greater technical expenditure with respect to signal processing devices, delivering better results. This is especially significant at the rate of rotation measurement accuracies required over the wide dynamic range required in inertial technology.
So-called resetting (closed loop) Sagnac interferometers are described in U.S. patent Ser. No. 4,705,399 as well as European patent publication EP-A-0,294,915. According to the teaching of the above-mentioned United States patent, phase resetting at the phase modulator is achieved by means of a rectangular modulated stepped phase shift with reverse jumps of 2 .pi. phase variation. The sawtooth pitch is a measure of the rotation rate. European patent application No. 90/100,103.2 discloses a substantially improved process in which a reduced rotation rate insensitivity range is achieved at low rates of rotation and scale factor drift is prevented in such a sensor arrangement.
In the process described in Ep-A-0,294,915, the photodetector signals are only evaluated at specified periods. Outside such periods the signals are separated by an electronic switch from the downstream signal processing device. A rate of rotation signal is obtained by means of a demodulator which controls the amplitude and phase relationship of phase modulation signals that are fed to the phase modulator in addition to the other modulation signals. Following amplitude, phase relationship, magnitude and sign, the additional modulation signals contain the reset-effective optical phase shift and are therefore also a direct measure of the rotation rate. Compared to the so-called phase ramp reset process described in the above-mentioned U.S. patent, is reset signal need not rise as a ramp due to the periodic readout of the photodetector via the above-mentioned switch since a more simply generated rectangular oscillation is sufficient.
A major technical problem of rotation rate sensors of the above-mentioned type has been sufficiently precise demodulation of the readout signal. On the one hand, the demodulators must possess a very large dynamic range (frequently greater than six decades), especially for open loop arrangements. On the other hand, they must be designed for frequencies of a few 100 kHz. As a rule, fast synchronous demodulators, constructed from FET switches, are used. However, error terms can occur due to charge carrier injection during the switch-over processes, offset errors caused by the introduction of interspersed harmonics of the gyro modulation signal into the input of the demodulator, phase differences between the modulation signal and the reference signal of the demodulator and so-called loss modulation. Moreover, both fast and high-resolution A/D converters are required Which are not yet current available.
In order to deal with these problems of gyro readout signal demodulation, European patent application No. 89/110,041.4, the content of which is hereby incorporated by reference, discloses a demodulation process that makes use of the fact that the large gyro bandwidth required for other reasons is not at all required for the gyro readout signal or rate of rotation signal. By suitable modulation of the generally very broadband light source (i.e., by pulse-controlled energization and deenergization of the light source), it is possible to transform the readout signal to a substantially lower frequency range. Although the phase modulator in the ring is, as previously, modulated with a modulation signal whose period is twice the light transit time in the ring, signals carrying rate of rotation information again occur at the photodetector. The carrier frequency of such signals is, however, considerably lower than the phase modulator frequency compared with those processes of the above-mentioned prior art. This is especially true for fiber optic gyros of more recent construction having shorter fiber lengths and, correspondingly, a high modulation frequency. The readout signal frequency reduction achieved for such devices is of decisive importance.